realizing the Advantages of the Air-Engine. 5 



5. Threefold Effect of Heat. — The communication of heat 

 to a substance produces, in general, three kinds of effects (set- 

 ting aside chemical, electrical, and magnetic phenomena, as 

 being foreign to the subject of the present paper) : — 



1st, An increase of temperature and expansive pressure ; 

 that is to say, an increased tendency to the communication of 

 heat to other bodies, and to the development of mechanical 

 power by expansion. 



2dly, A change of volume ; which, under a constant pres- 

 sure, is an increase for every substance, except some liquids 

 near their freezing points. 



Sdly, A change of molecular condition ; as from the solid 

 to the liquid state, or from the liquid or solid to the gaseous 

 state, or any imperceptible change of molecular arrangement ; 

 the change to the gaseous state being always accompanied by 

 an increase of volume. 



The heat which produces the first of those effects is known 

 by the name of sensible heat, as retaining the form of heat, 

 and, in short, making the body hotter. 



The heat which produces the second and third of those ef- 

 fects is called latent heat, as having disappeared in developing 

 a mechanical effect, and being capable of reproduction by re- 

 versing the change which caused it to disappear. 



Changes of volume are in general accompanied by changes 

 of molecular arrangement or condition, perceptible or imper- 

 ceptible. The latent heat of expansion, or of evaporation^ 

 therefore, as the case may be, consists partly of heat which 

 disappears in overcoming external pressure, and partly of that 

 which disappears in overcoming the mutual attraction of the 

 particles of the body. 



The latter forms by far the greater part of the latent heat 

 of evaporation. For example, as already stated, there dis- 

 appears, in evaporating one pound of water at 212°, a quantity 

 of heat equivalent to 



746,910 foot-pounds. 



The pressure of the steam produced is 2116*4 lb. on the 

 square foot. Its volume is not known exactly by experiment, 

 but is probably about 26^ cubic feet more than that of the 

 liquid water. Multiplying these two quantities together, it 



